The regulatory role of thymus-derived (T) cells on the antibody response is being investigated in chickens. One population of such T cells has been shown previously to be dependent on the bursa of Fabricius developmentally. Transfer studies of t cells from embryonically bursectomized and normal birds to thymectomized-irradiated animals are being used to study some characteristics of these cells. Labelling studies are being conducted to ascertain if the population of regulatory (suppressor) cells has a different life span than other T cells populations. The presence or absence of the several populations of T cells in chickens is being determined by studies employing a range of thymus-dependent and thymus-independent antigens. The distribution of these various populations in various lymphoid organs of the chicken is also under investigation through transfer studies. In addition, the potential role of the regulatory cells in the development of an autoimmune process will be studied using the spontaneous autoimmune thyroiditis occuring in obese strain chickens as a model.